1. Field of the Invention
The present invention relates to a gas sensor apparatus and to a concentration measurement method using the gas sensor.
2. Description of the Related Art
In an internal combustion engine such as a diesel engine or a gasoline engine, an EGR control for returning exhaust gas to the intake gas side is generally performed so as to reduce fuel consumption and purify exhaust gas. In order to perform the EGR control, a gas sensor is used to measure the proportion of a specific gas component (e.g., oxygen) contained in the intake gas or the proportion of the specific gas component contained in the exhaust gas.
The gas sensor includes a sensor element disposed in a gas (i.e., a target for measurement). The sensor element outputs a value representing the concentration of a specific gas component (e.g., oxygen concentration), which is the ratio of the specific gas component. However, the value output from the sensor element is known to be affected not only by the concentration of the specific gas component of the gas but also the pressure of the gas.
In recent years, since the degree of fineness of control of an internal combustion engine has increased, there has been an increasing need for measuring the concentration of a specific gas component more accurately. In order to measure the concentration of a specific gas component of a gas (i.e., a target for measurement) more accurately, various methods have been proposed for eliminating the influence of the gas pressure upon the value output from the gas sensor (see, for example, Patent Document 1).
Patent Document 1 discloses a configuration including a sensor element which measures the concentration of a specific gas component of a gas (i.e., a target for measurement) and a pressure sensor which measures the pressure of the gas. In addition, Patent Document 1 proposes a method (correction method) for eliminating the influence of the gas pressure on the value which is output from the gas sensor and which represents the concentration of the specific gas component. In this method, the value output from the gas sensor is multiplied by a coefficient based on the pressure measured by the pressure sensor, whereby the influence of the gas pressure on the output value is eliminated.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2013-036852
3. Problems to be Solved by the Invention
The method described in the above-described Patent Document 1 gives rise to the following problem. In the case where a change in pressure with time is relatively small (in the case of a static pressure change), the influence of pressure on the value output from the gas sensor can be eliminated easily. However, in the case where a change in pressure with time is relatively large (in the case of a dynamic pressure change), the influence of pressure on the value output from the gas sensor cannot be completely eliminated, and the accuracy in measuring the concentration of the specific gas component is greatly deteriorated.